"Canonical has today publicly confirmed that they are working on a new cross-platform displayer server for Ubuntu. Called 'Mir', the X Window Server replacement is tasked with 'enabling development of the next generation Unity'. Which, in yet another about-turn, is to be rebuilt in Qt/QML." It'll be used for all Ubuntu variants (phone, tablet, desktop), and the first version will be released come May.

They discussed this in the MirSpec page.
I don't know if/how valid it is but below are the excerpt.

"Why Not Wayland / Weston?

An obvious clarification first: Wayland is a protocol definition that defines how a client application should talk to a compositor component. It touches areas like surface creation/destruction, graphics buffer allocation/management, input event handling and a rough prototype for the integration of shell components. However, our evaluation of the protocol definition revealed that the Wayland protocol suffers from multiple problems, including:

1.The input event handling partly
recreates the X semantics and is thus likely to expose similar problems to the ones we described in the introductory section.

2. The shell integration parts of the protocol are considered privileged from our perspective and we'd rather avoid having any sort of shell behavior defined in the protocol.

However, we still think that Wayland's attempt at standardizing the communication between clients and the display server component is very sensible and useful, but it didn't fit our requirements and we decided to go for the following architecture w.r.t. to protocol-integration:

1. A protocol-agnostic inner core that is extremely well-defined, well-tested and portable.

2. An outer-shell together with a frontend-firewall that allow us to port our display server to arbitrary graphics stacks and bind it to multiple protocols.

In summary, we have not chosen Wayland/Weston as our basis for delivering a next-generation user experience as it does not fulfill our requirements completely. More to this, with our protocol- and platform-agnostic approach, we can make sure that we reach our goal of a consistent and beautiful user experience across platforms and device form factors. However, Wayland support could be added either by providing a Wayland-specific frontend implementation for our display server or by providing a client-side implementation of libwayland that ultimately talks to Mir."